1. Field of the Invention
The present invention relates to a method for reproducing signals recorded by magnetic orientation states in a magnetic material of a magnetic recording medium and an apparatus therefor.
2. Related Background Art
Magnetic recording mediums on which information is recorded by magnetic orientation states in a magnetic material such as magnetic recording mediums and magneto-optical mediums are attracting attention as rewritable high-density recording mediums. In recent years, still higher recording density of the magnetic recording medium is demanded for larger capacity of the recording medium.
In a magneto-optical recording system employing a magneto-optical medium and a recording-reproducing apparatus therefor, information is recorded by forming magnetic domains on a magnetic thin film by thermal energy of a semiconductor laser, and the recorded information is read out by utilizing magneto-optical effect. Generally, the linear recording density of an optical recording medium depends greatly on the laser wavelength of the reproducing optical system, and the numerical aperture NA of the objective lens. Specifically, the laser wavelength xcex and the numerical aperture NA of the objective lens of the reproducing optical system decide the diameter of the beam waist, whereby the detectable range of the spatial frequency of the recording pits is limited to about 2NA/xcex.
For achieving higher recording density with a conventional optical disk, the laser wavelength should be shorter, or the NA of the objective lens should be larger in the reproducing optical system. However, the laser wavelength xcex cannot readily be shortened owing to the efficiency limit and heat generation of the laser element, and the increase of the numerical aperture NA of the objective lens results in smaller focal depth, requiring higher mechanical accuracy, disadvantageously.
To solve the above problems, super-resolution techniques are being developed to improve recording density by changing the constitution of the recording medium and changing the reproducing method without changing the laser wavelength and the numerical aperture of the objective lens.
Japanese Patent Application Laid-Open No. 3-93058 discloses a signal-reproducing method. In this method, a multilayered film having a displacement layer and a memory layer coupled magnetically is provided, signals are recorded on the memory layer, the magnetization orientation in the displacement layer is made uniform, a laser light beam is projected thereto for heating, and the signals recorded on the memory layer is transferred to the heated region of the displacement layer for reading the recorded signals.
In this method, the size of the region heated by the laser up to the signal transfer temperature for signal detection can be made smaller than the laser spot diameter, whereby interference between the signs can be decreased to enable reproduction of signals of a cycle less than optical diffraction limit.
In any of the known super-resolution systems, the reproducing light is partly intercepted with a mask to limit the pit-reading aperture to a smaller region to improve the resolution limit. Thereby, the light intercepted by the mask is not utilized, and the amplitude of the reproduced signal is decreased, disadvantageously. In other words, the portion of the light intercepted by the mask does not contribute to the signal reproduction. Therefore, the smaller the aperture for higher resolution, the less is the effective light, and the lower is the signal level.
To solve the above problems, the inventors of the present invention disclosed a method for reproducing high-density recorded signal in Japanese Patent Application Laid-Open No. 6-290496 in which a special magnetic recording medium is employed, a magnetic domain wall existing at the border of the recorded mark is displaced by temperature gradient to the higher temperature side, and the domain wall displacement is detected to reproduce the high-density recorded signal.
In this method, however, since the temperature gradient is formed by heating the recording medium with the reproducing light beam itself, the peak of the temperature distribution is formed inside the reproducing light spot, and the displacement of the domain wall from the front side of the displacement of the region of the domain wall displacement and that from the rear side thereof are both read by the reproduction spot, not giving satisfactory signal reproduction. Therefore a separate means for controlling the temperature distribution is required in addition to the reproducing light beam, which complicates the reproduction apparatus.
FIG. 1 shows a constitution of a conventional system. In FIG. 1, magneto-optical disk 101 is constituted of substrate 102, magneto-optical medium 103 formed thereon, and protection layer 104 formed further thereon. Substrate 102 is formed from glass or a plastic material. Magneto-optical medium 103 is comprised of a multiple layer comprising at least a memory layer and a displacement layer, and is capable of reproducing record marks of less than optical diffraction limit of the optical system by displacing a domain wall by utilizing temperature gradient caused by light beam irradiation without changing recorded data in the memory layer, magnetizing uniformly and almost entirely the reproducing light beam-spotted region on the displacement layer, and detecting the change of polarization direction of the reflected light beam. Magneto-optical disk 101 is set to a spindle motor by a magnet chucking or a like means to be rotatable on a rotation axis.
Parts 105 to 117 constitute an optical head for projecting a laser beam to magneto-optical disk 101 and for receiving information from reflected light. The parts comprise condenser lens 106 as an objective lens, actuator 105 for driving condenser lens 106, semiconductor laser 107 of a wavelength of 680 nm for record reproduction, semiconductor laser 108 of wavelength of 1.3 xcexcm for heating, collimator lenses 109,110, dichroic mirror 111 for completely transmitting light of 680 nm and completely reflecting light of 1.3 xcexcm, beam splitter 112, dichroic mirror 113 for intercepting light of 1.3 xcexcm and completely transmitting light of 680 nm to prevent leakage of light of 1.3 xcexcm into the signal detecting system, xcex/2 plate 114, polarized light beam splitter 115, photosensors 117, condenser lenses 116 for photosensor, differential amplification circuit 118 for differentially amplifying the condensed and detected signals for respective polarization direction, LD driver 119, and controller 120 for recording power control.
The laser beams of 680 nm and 1.3 xcexcm emitted respectively from semiconductor lasers 107,108 for recording-reproducing and heating are introduced through collimator lenses 109,110, dichroic mirror 111, beam splitter 112, and condenser lens 106 to magneto-optical disk 101. Condenser lens 106 moves in the focusing direction and the tracking direction under control by actuator 105 to focus the laser beams successively on magneto-optical medium 103 by tracking along a guiding groove formed on magneto-optical disk 101. The light flux of 1.3 xcexcm is made smaller than the aperture diameter of condenser lens 106 to make the NA smaller than that of the light of 680 nm which is condensed through the entire area of the aperture.
The heating spot, which is formed with a larger wavelength and a smaller NA, has a larger diameter of heating beam than the recording-reproducing spot of recording-reproducing beam as shown in FIGS. 3A and 3B. Thereby, a desired temperature gradient is produced in the recording-reproducing spot region on the moving medium face as shown in FIG. 3D. The laser beam reflected by magneto-optical disk 101 is deflected by beam splitter 112 to the optical path toward polarized light beam splitter 115, and travels through dichroic mirror 113, xcex/2 plate 114, and polarized light beam splitter 115. The split light beams are respectively condensed by lenses 116 onto sensors 117 corresponding to magnetization polarity of the spot on magneto-optical layer. The condensed light beams are composed only of 680 nm light since dichroic mirror 113 intercepts the 1.3 xcexcm light. The outputs from the respective photosensors 117 are amplified differentially by differential amplifier 118 to output the magneto-optical signals. Controller 120 receives information on rotation rate of magneto-optical disk 101, recording radius, recording sectors, and so forth and outputs recording power, and recording signals to control LD driver (laser diode driver) 119, and magnetic head driver 124. LD driver 119 drives semiconductor lasers 107,108. In this example, LD driver 119 supplies a recording power and a reproducing power to semiconductor laser 107, and supplies a heating beam power to semiconductor laser 108.
Magnetic head 123 applies a modulation magnetic field onto the laser irradiation spot on magneto-optical disk 101 for the recording operation. Magnetic head 123 is placed in opposition to condenser lens 106 with interposition of magneto-optical disk 101. During recording, recording-reproducing semiconductor laser 107 applies recording laser power by DC light irradiation under control by LD driver 119, and synchronously magnetic head 123 produces magnetic fields of different polarities under control by magnetic head driver 124 in correspondence with the recording signals. Magnetic head 123 moves with the optical head in a radius direction of magneto-optical disk 101, and applies a magnetic field successively on recording onto the laser irradiation site of magneto-optical medium 103. The magneto-optical medium 103 is constituted of three layers, as shown in FIG. 3C, comprising a memory layer, a switching layer, and a displacement layer, and have respectively a magnetic domain wall structure as shown by the arrow marks.
The recording-reproducing operation is explained by reference to FIGS. 2A to 2F. FIG. 2A shows recording signals, FIG. 2B a recording power, FIG. 2C modulating magnetic fields, FIG. 2D record marks, FIG. 2E reproducing signals, and FIG. 2F binary signals. In recording of the recording signals as shown in FIG. 2A, the power of semiconductor laser 107 is controlled to be at a prescribed level during the recording operation, and modulating magnetic field is applied in accordance with the recording signals. Thereby, record mark sequence is formed in the process of cooling of the magneto-optical medium, as shown in FIG. 2D, where the line-shadowed portions are magnetic domains magnetized in the direction corresponding to the record marks in the present invention, and the white blank portions are magnetic domains magnetized in the reverse direction thereto.
The reproduction operation is explained below by reference to FIGS. 3A to 3D. The displacement layer 76 is heated by a heating beam 74 up to a temperature for causing the displacement of the domain wall in the displacement layer of the medium. The isothermal line 75 of the temperature Ts of the recording medium, which is the main factor for inducing displacement of the domain wall, crosses the beam movement direction 71 both in the front portion and in the rear portion of the beam spot. The domain walls can displace backward from the front side and forward from the back side of the beam movement direction as shown by numeral 72 in FIG. 3A. Therefore, the magnetic domain wall displacement signals from the front side only can be detected by placing record-reproducing beam 73 only at the front side of the beam-moving direction as shown in FIG. 3A. Similarly, the magnetic domain wall displacement signals from the back side only can be detected by placing record-reproducing beam 73 at the back side of the beam-moving direction as shown in FIG. 3B. In FIG. 3C, numeral 77 denotes a switching layer, and 78 a memory layer.
In either case, the record mark sequence as shown in FIG. 2D is reproduced by the record-reproducing beam to obtain reproduced signals (FIG. 2E), and further to obtain binary signals (FIG. 2F). In the above magneto-optical recording-reproducing method, a light beam is projected to cause displacement of the domain walls of the record marks in the displacement layer by utilizing temperature gradient caused by the light beam without change of the recorded data in the memory layer, and the change of the polarization direction of the reflected light beam is detected to reproduce the record marks. According to this magneto-optical recording-reproducing method, the magnetization states carried by the reproducing beam are all equal as shown in FIGS. 3A and 3B. Therefore, the reproduced signals are rectangular, and record marks of less than diffraction limit of the optical system can be reproduced without decreasing the reproducing signal amplitude. Thereby, a medium and a method for magneto-optical recording can be provided which have been improved in recording density and transfer rate.
An object of the present invention is to solve the aforementioned problems of the disclosed method of reproduction of high-density record signals.
Another object of the present invention is to provide an improved method of reproduction of information signals of a domain wall displacement detection type, in which high-density record signals are obtained by heating the magneto-optical medium with the reproducing light beam itself without complicating the reproduction apparatus.
A further object of the present invention is to provide an apparatus for reproducing the information signals of a domain wall displacement detection type recorded on a recording medium.
The signal reproducing method of the present invention for reproducing information by domain wall displacement on a recording medium having recorded information comprises steps of projecting a light spot onto the recording medium to cause temperature distribution thereon; moving relatively the light spot and the recording medium; applying a reproducing magnetic field to the light spot area on the recording medium to prevent displacement of the domain wall from the rear portion of the moving light spot into the inside thereof, and detecting the domain wall displacement to reproduce the information.
The signal reproducing apparatus of the present invention for reproducing information by domain wall displacement on a recording medium having recorded information comprises a means for projecting a light spot onto the recording medium to cause temperature distribution thereon; a means for moving relatively the light spot and the recording medium; a means for applying a reproducing magnetic field to the light spot area on the recording medium to prevent displacement of the domain wall from the rear portion of the moving light spot into the inside thereof, and a means for detecting the domain wall displacement to reproduce the information.